1. Field of the Invention
The present invention relates to a belt-drive control device, a color-shift detecting method, a color-shift detecting device, and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, as described in Japanese Patent Application Laid-Open No. 2000-310897, when a transfer conveying belt is rotated by driving a drive roller at a fixed pulse rate, based on a position to be detected due to a belt mark on the transfer conveying belt, a speed profile that cancels a speed fluctuation Vh expected to occur due to a known thickness profile across the whole circumference of the transfer conveying belt is measured in advance, and at a pulse rate modulated with respect to the speed profile, a drive motor control signal is generated, and based on this, the motor is driven to drive the transfer conveying belt via the drive roller, whereby the final speed Vb of the transfer conveying belt is made free from fluctuation.
Color-shift detecting methods to detect positional deviations of images in a plurality of colors in a color image forming apparatus are disclosed in, for example, Japanese Patent No. 2573855, Japanese Patent Application Laid-Open No. H11-65208, Japanese Patent Application Laid-Open No. H11-102098, Japanese Patent Application Laid-Open No. H11-249380, and Japanese Patent Application Laid-Open No. 2000-112205. In these color-shift detecting methods, near the respective ends in the width direction of a transfer conveying belt that supports a transfer sheet and is conveyed along arrangement of a plurality of photoconductor drums and transfers toner images in the colors on the photoconductor drums onto the transfer sheet, toner marks in the colors are formed in a predetermined alignment pattern, and a pair of light sensors read the toner marks on the ends of the transfer conveying belt and based on the read signals, calculate the positions of the marks in the mark alignment (pattern). Then, deviation amounts in the vertical scanning direction (transfer conveying belt moving direction) and deviation amounts in the horizontal scanning direction (width direction of the transfer conveying belt) of the images in the colors from the reference position, a deviation amount of the effective line length of the horizontal scanning line, and skew of the horizontal scanning line are calculated.
Japanese Patent Application Laid-Open No. H11-231586 describes an image forming apparatus in which resist marks in black and yellow are formed on a transfer conveying belt and a parallel line pattern is formed in parallel to the resist marks, a photoelectric sensor detects straight line portions of the resist marks and temporarily detects a position deviation amount of an image in yellow from an image in black from the detected values, and when speed fluctuation is detected from the detection results of the resist marks by the photoelectric sensor, the temporary position deviation amount is corrected based on the speed fluctuation in a corresponding detecting section, and similar processing is also executed for position deviation amounts in magenta and cyan, and based on these corrected values, the writing positions of the images in the colors except for black are corrected to correct color shifts.
Japanese Patent Application Laid-Open No. 2004-123383 describes a belt driving method in which a rotation angular displacement or a rotation angular velocity of a driven roller, and from the detection results, an alternate current component of the rotation angular velocity including frequencies corresponding to periodical thickness fluctuation in the circumferential direction of the belt is detected, and based on an amplitude and a phase of this alternate current component, the rotation of the drive roller is controlled.
Japanese Patent Application No. 2004-161416 describes a color-shift detecting method to prevent harmful influences on color shift correction accuracies by eliminating fluctuation components caused by the belt thickness by arranging the plurality of mark set groups on the belt at 360 degrees/the number of mark sets.
Typical methods of color image forming include a direct transfer method in which toner images in different colors formed on a plurality of photoconductors are directly transferred onto a transfer sheet while superposing these, and an intermediate transfer method in which toner images in different colors formed on a plurality of photoconductors are transferred onto an intermediate transfer member such as an intermediate transfer conveying belt while superposing these and then collectively transferred onto a transfer sheet. These are called tandem-type since a plurality of photoconductors are arranged to face a transfer sheet or an intermediate transfer member, wherein, electrophotography processing such as electrostatic latent image forming and developing are applied to each of yellow (hereinafter, referred to as Y), magenta (referred to as M), cyan (referred to as C), and black (referred to as K) on each photoconductor to form toner images in the colors, and the toner images in the colors are transferred onto a transfer sheet while traveling in the direct transfer method or onto an intermediate transfer member while traveling in the intermediate transfer method.
Generally, in tandem-type color image forming apparatuses using these methods, in the case of the direct transfer method, an endless belt that travels while supporting a transfer sheet is used as a direct transfer conveying belt, and in the case of the intermediate transfer method, an endless belt that receives images from photoconductors and carries the images is used as an intermediate transfer conveying belt. An image forming unit including four photoconductors is arranged on one side in the traveling direction of the endless belt.
In the tandem-type color image forming apparatuses, accurate transferring of toner images in colors onto a transfer sheet as a transfer medium or an intermediate transfer conveying belt by a transfer unit is important to prevent color shifts. Therefore, in any transfer method, to prevent color shifts due to speed fluctuation of the direct transfer conveying belt or the intermediate transfer conveying belt, it is effective that an encoder is attached to one of a plurality of follower shafts that follow the direct transfer conveying belt or the intermediate transfer conveying belt of the transfer unit, and according to rotation speed fluctuation of this encoder, the rotation speed of a drive roller that drives the direct transfer conveying belt or intermediate transfer conveying belt is controlled by means of feedback.
However, it is difficult to completely eliminate the speed fluctuation of the direct transfer conveying belt or intermediate transfer conveying belt although it can be reduced by means of feedback control. Therefore, it is desirable for improvement in image quality that a color-shift detecting device is also mounted while feedback control of the direct transfer conveying belt or intermediate transfer conveying belt is performed to reduce the speed fluctuation of the direct transfer conveying belt or intermediate transfer conveying belt.
As most general method to realize feedback control, proportional control (PI control) is available. According to this, a position error e(n) is calculated from a difference between a target angular displacement Ref(n) of the encoder and a detection angular displacement P(n−1) of the encoder, and a low-pass filter is applied to the calculation results to eliminate high-frequency noise and control gain is applied, and the result is added with a constant standard drive pulse frequency to control a drive pulse frequency of a drive motor connected to the drive roller, whereby performing control so that the encoder is always driven with the target angular displacement.
As actual feedback control, by using an encoder pulse counter that counts rise edges of output pulses of the encoder and a control cycle counter that counts for each control cycle (for example, 1 millisecond), from a difference between the calculation results of a target angular displacement of movement during the control cycle (1 millisecond) and a detection angular displacement obtained by acquiring the count of the encoder pulse counter for each control cycle, a position error is acquired.
Detailed calculation when the roller diameter of the follower shaft to which the encoder is attached is set to φ15.615 is as followse(n)=θ0*q−θ1*ne[rad]where e(n) is position error (calculated by the current sampling), θ0 is moving angle per control cycle (=2π*V*E−3/15.565π [rad]), θ1 is moving angle per 1 pulse of encoder (=2π/p [rad]), q is count of control cycle timer, and ne is count of encoder pulse counter for each control cycle.
Assuming that an encoder with resolution of 300 pulses per rotation in a control cycle of 1 ms is used and an endless belt (direct transfer conveying belt or intermediate transfer conveying belt) is controlled by means of feedback so as to move at 162 mm/s, calculation is as followsθ0=2π*162*E−3/15.615π=0.0207487[rad]θ1=2π/p=2π/300=0.0209439[rad]
A position error is acquired by performing the calculation for each control cycle and feedback control is performed.
However, this method causes changes in conveying speed of the transfer sheet or the intermediate transfer conveying belt due to a minute thickness of the endless belt, and causes lowering in image quality such as image deviation from an ideal position and positional displacement of an image among a plurality of recording sheets, resulting in deteriorated repetitive image position reproducibility among recording sheets.
As a reason, at an endless belt driving position, when it is assumed that the conveying speed of the endless belt is determined at the center in the thickness direction of the endless belt, the conveying speed V of the endless belt is as followsV=(R+B/2)×ω  (1)where R is drive roller radius, B is belt thickness, and ω is drive roller angular velocity. However, if the belt thickness B fluctuates, the position of the thickness effective line of an endless belt 701 to be driven by a drive roller 702 changes as shown in FIG. 20. This means that the belt drive effective radius changes, and (R+B/2) of Equation (1) changes, so that the conveying speed of the endless belt 701 changes even when the angular velocity ω of the drive roller 702 is constant. Namely, even when the drive roller 702 is rotated at a constant angular velocity, if the endless belt 701 fluctuates in thickness, the conveying speed of the endless belt 701 changes.
FIG. 21 depicts a model of a belt drive conveying system. The endless belt 701 is laid across the drive roller 702 and driven rollers 703 and 704.
First, thickness fluctuation and conveying speed fluctuation in one round of the endless belt 701 when the drive shaft of the drive roller 702 is rotated at a constant angular velocity are schematically shown in FIG. 22. When a thick portion of the endless belt 701 is wound around the drive shaft, the belt drive effective radius shown in FIG. 20 is increased, and the belt conveying speed is increased. To the contrary, when a thin portion of the endless belt 701 is wound around the drive shaft, the belt conveying speed lowers.
FIG. 23 depicts belt thickness fluctuation on the follower shaft when the endless belt 701 is conveyed at a constant conveying speed and belt conveying speed fluctuation detected on the follower shaft of the driven roller 703, and FIG. 24 depicts an example of counts of the encoder pulse counter that counts main pulses of the encoder attached to the driven roller 703. Even when the endless belt 701 is ideally conveyed without speed fluctuation, if the thick portion of the endless belt 701 is wound around the follower shaft of the driven roller 703, the belt following effective radius is increased, and the rotation angular velocity of the follower shaft of the driven roller 703 is decreased. This is detected as conveying speed lowering of the endless belt 701. When the thin portion of the endless belt 701 is wound around the follower shaft of the driven roller 703, the rotation angular velocity of the follower shaft of the driven roller 703 is increased and is detected as an increase in belt conveying speed.
When the belt thickness fluctuation occurs and the belt conveying speed is detected by the encoder based on the rotation angular displacement of the follower shaft, erroneous detection components are generated. Therefore, even when the endless belt 701 is conveyed at a constant speed, due to thickness fluctuation of the endless belt 701, in the rotation angular displacement detection of the follower shaft, it is detected as if the endless belt 701 fluctuates in speed. Therefore, in feedback control to detect the belt conveying speed based on the follower shaft rotation angular displacement as conventionally, the speed fluctuation due to fluctuation in thickness of the belt cannot be controlled.
A method of solving the belt speed fluctuation due to belt thickness fluctuation is described in Japanese Patent Application Laid-Open No. 2000-310897. As a feature of the method, when a drive roller is driven at a constant pulse rate, based on a position to be detected by a belt mark on the belt, a speed profile that cancels speed fluctuation Vh expected to occur due to the known thickness profile across the whole circumference of the belt is measured in advance, a drive motor control signal is generated at a pulse rate modulated with respect to the speed profile, and based on this, the motor is driven to drive the belt via the drive roller, whereby the final belt speed Vb is made free from fluctuation.
However, in the above method, as speed profile data, data for each control cycle is needed, so that when the control cycle is short, a memory with a high capacity becomes necessary, and when the control cycle is set long, feedback control itself cannot show a sufficient effect. For example, when the belt circumferential length is 815 mm, the belt driving speed is 125 mm/s, and the control cycle is 1 ms, the control is performed 6520 times per one round of the belt as shown below.815 mm/(125 mm/s×1 ms)=6520 times
When a data size of a belt thickness per one point is expressed by 16 bits, a memory with a capacity equal to or more than 100 kilobits becomes necessary as shown below.6520×16 bits=104320 bits
Therefore, when this control is performed in an actual apparatus, a belt thickness profile storage memory that stores speed profile data must be newly prepared as a nonvolatile memory, and even if the speed profile data is compressed and stored in the nonvolatile memory and is decompressed in a volatile memory when turning the power source on, a high-capacity memory is still necessary. Therefore, a separate memory is necessary in addition to the memory that has been used as a normal work area, and this results in a remarkable cost increase and is not realistic.
Furthermore, according to the above method, the belt thickness itself must be measured as belt thickness profile data, and as means for this, a laser displacement gauge is used to measure the belt thickness. The measurement data is inputted from an input unit such as an operation panel when shipping the product or by service man.
However, to measure the thickness fluctuation of several micrometers of the belt, a high-accuracy measuring unit is necessary, and data management and data amount of the measured results are large in size, and this may cause an input error.
In the color-shift detecting device, since speed fluctuation of the belt due to belt thickness fluctuation cannot be controlled by means of feedback control in that the belt conveying speed is detected based on the conventional follower shaft rotation angular displacement, the fluctuation harmfully influences the color shift correction accuracies and causes image deterioration. Namely, the belt speed when mark sets for color shift correction are drawn on the belt and the belt speed when the mark sets are detected are different from the belt speed when images are actually drawn on the belt, resulting in color shifts. As seen in the color-shift detecting method described in Japanese Patent Application No. 2004-161416, the plurality of mark sets are arranged at 360 degrees/the number of mark set groups so as to eliminate the belt thickness fluctuation components and prevent harmful influences on the color shift correction accuracies. However, in this case, the length of all groups of mark sets may become long, the color shift correction time may become slightly long, or the toner consumption may slightly increase.
Furthermore, if the execution timing of color shift detection and correction is only either one of an execution timing instructed through an operation panel or an automatic execution timing, image quality that a customer desires cannot be provided in a desired timing in some cases.
The belt thickness has a sine-wave-shaped profile in most cases, so that when high-resolution measurement can be made with an external jig, the external jig calculates a phase and a maximum amplitude based on belt marks of a follower shaft rotation angular displacement detection error from the measuring results of the belt thickness, and stores the calculation results in a nonvolatile memory, and by using the results as control parameters and inputting them from an operation panel mounted on an actual apparatus, feedback control of the belt drive motor can be realized. However, if calculation of the phase and maximum amplitude to be stored in the nonvolatile memory is performed by only either one of manual execution or automatic execution, in some cases, image quality that a customer desires cannot be provided in a desired timing. Particularly, when the phase and maximum amplitude of the angular displacement error change due to aging of the belt, color shift detection and correction makes worse the color shifts unless the calculation is performed.